Stricter chord parsing

[alexanderkoller]

Ensure that only chords that follow the AABA notation conventions are accepted by the parser. For instance, the ABC file below contains a chord "D79" which is incorrectly parsed as a 7/9 chord over the root note "D7", which of course causes problems.

%abc-2.1
X:1
T:Zip a dee doo dah
C:Allie Wrubel
M:4/4
L:1/8
Q:1/4=180
K:C
V:Ld octave=-1 middle=B,  clef=treble-8   name="Ld"  snm=""
"C" E,2 z  E, F,2 G,2-| G,2 c,6 | "F" A,2 z A, F,2 "C" G,2-|
 G,8 | "F" c,4 c,4 | "C6" A,2 G,2 E,2 C,2 | 
"D79" E,2 C,2 E,2 "G7" D,2-| D,8| "C" E,3 E, F,2 G,2-|
G,2 c,6| "F" A,3 A, F,2 "C" G,2-| G,8| 
"F" A,2 z  B, c,2 "C" e,2- | e,2 "Am" c,6| "Dm" d,2 z "G7" d, A,2 "C" c,2-|c,8